The present invention relates generally to getter pumps and, more particularly, to a getter pump with high gas sorption velocity.
In the field of vacuum technology so-called "getter pumps" have been known for several decades. Getter pumps are static devices, i.e., devices that operate without moving parts. The operation of getter pumps is based on the chemisorption of all gases, except for noble gases, by nonevaporable getter materials, which are sometimes referred to as "NEG materials." The primary NEG materials are titanium-based and zirconium-based alloys.
The active members of getter pumps are referred to as "getter elements." In some getter pumps the getter elements are formed by depositing a layer of getter material having a thickness of less than one millimeter onto generally flat metal supports. These getter pumps are widely used but suffer from the disadvantage that their gas sorption capacity is relatively small because the getter elements contain a relatively small amount of getter material. To overcome this drawback, SAES Getters S.p.A., the assignee of the present application, has recently introduced getter pumps having increased gas sorption capacity in which the getter elements consist of porous bodies formed by sintering powdered NEG material. Examples of such getter pumps are disclosed in U.S. Pat. No. 5,320,496 to Manini et al. and U.S. Pat. No. 5,324,172 to Manini et al., both of which are assigned to SAES Getters S.p.A.
FIG. 1 shows the getter pump disclosed in U.S. Pat. No. 5,320,496 to Manini et al. This pump includes a plurality of getter elements 60 stacked in a cylindrical metal chamber 62. The getter elements 60 occupy the peripheral portion of the chamber while leaving a cylindrical cavity in the center of the chamber. A heater 64 for activating the getter material and maintaining the getter material at its optimal working temperature is provided in the cavity in the center of the chamber.
FIGS. 2a and 2b show the getter pump disclosed in U.S. Pat. No. 5,324,172 to Manini et al. This pump includes a plurality of rectangular getter elements 70 having broad surfaces 70a and narrow surfaces 70b disposed in the peripheral portion of a cylindrical metal chamber 72. A heater 74 for activating the getter material and maintaining the getter material at its optimal working temperature is provided in the center of the chamber. As shown in FIG. 2b, getter elements 70 are arranged around the center of the chamber such that narrow surfaces 70b face heater 74.
The design of the above-described getter pumps shown in the '496 and '172 patents provides high gas sorption capacity. In some applications, however, gas sorption velocity is more important characteristic than gas sorption capacity. The design of the getter pumps shown in the '496 and '172 patents is not conducive to high gas sorption velocity because of reduced gas conductance or inefficient heating of the getter elements. In particular, with reference to FIG. 1, in the getter pump shown in the '496 patent there is reduced gas conductance in the region between the inner wall of housing 62, which defines a chamber, and the stack of getter elements 60. As such, the outermost surfaces of getter elements 60 (relative to the center of the chamber) are not readily accessible to gases in the chamber. Referring now to FIGS. 2a and 2b, in the getter pump shown in the '172 patent the configuration of rectangular getter elements 70 enables conveyance of gases onto broad surfaces 70a and narrow surfaces 70b of elements 70. This configuration, however, inefficiently heats getter elements 70 because radiative heat from heater 74 directly contacts substantially only narrow surfaces 70b. Thus, the other portions of getter elements 70 are primarily indirectly heated by the conduction of heat within each element. The reduced gas conductance and inefficient heating of the getter elements in the getter pumps shown in the '496 and '172 patents contribute to reduce the frequency with which gas molecules collide effectively with the surfaces of the getter elements that are most peripheral with respect to the center of the pump, thus resulting in reduced overall gas sorption velocity.
Other getter pumps that use getter elements that consist of porous bodies formed by sintering powdered NEG material are known, but these pumps are generally optimized for specific applications. For example, European Patent Publication No. EP 0 753 663 A1 discloses a getter pump in which a set of disk-shaped getter elements are supported on a central mounting having a heater housed therein. This pump is intended for use in portable instruments and, therefore, its design objective is to obtain adequate performance in a small pump with low power requirements for heating the getter elements. This pump does not retain its desirable characteristics when produced with larger dimensions.
International Publication No. WO 96/17171 discloses a getter pump for use in a semiconductor processing chamber. This pump includes a plurality of disk-shaped getter elements supported on a support element, e.g., a metal rod, that is substantially parallel to the floor of the processing chamber. The getter pump is placed directly within the processing chamber without a housing, but a substantially L-shaped thermal shield that partially surrounds the getter elements may be provided. As a result of the excellent gas conductance to the surfaces of the getter elements, this pump has high gas sorption velocity. Unfortunately, this getter pump is specifically designed for use in semiconductor processing chambers and, consequently, may not be suitable for use in other applications such as, for example, applications involving scientific instruments in which the getter pump is connected to a space to be evacuated via a suitable conduit.
In view of the foregoing, there is a need for a getter pump with high gas sorption velocity that is suitable for use in a variety of applications.